1. Field of the Invention
The present invention relates to a collimator adapted to convert divergent light beams outputted from each of optical fibers of an optical fiber bundle to parallel light beams through a collimator lens. More particularly, the invention relates to a collimator having a structure in which a large diameter optical fiber is disposed between the collimator lens and an optical fiber bundle obtained by tying a large number of multimode optical fibers in a bundle serving as a single optical transmission path, and also relates to an optical filter device incorporating the collimator at an incident side thereof, and to an optical measuring apparatus incorporating the collimator at an incident side thereof.
2. Related Art
An optical fiber collimator is a device adapted to convert divergent light beams outputted from an optical fiber 10 to parallel light beams through a collimator lens 12, as shown in FIG. 6A. The optical fiber collimator obtains parallel light beams by placing a light output end face of the optical fiber 10 at a focal position of the collimator lens 12. Such an optical fiber collimator is widely used, for example, in a case where various optical filters are inserted into a transmission optical system including optical fibers. For instance, in a case where a wavelength selection filter utilizing interference due to dielectric multilayer film is inserted into the transmission optical system including optical fibers, two of the optical fiber collimators are placed opposite to each other and are aligned with each other. Then, an interference film filter is inserted between both the optical fiber collimators (see, for example, JP-A-62-75606). With this configuration, light having been incident upon one of an optical fiber of one of the optical fiber collimators changes in intensity at a predetermined wavelength according to the spectral transmission characteristic of the optical filter and is coupled to an optical fiber of the other optical fiber collimator.
Such an optical fiber collimator has been used without problems in a transmission system using an infrared single-mode optical fiber oramultimode optical fiber, such as a GI50 optical fiber, which are mainly used in optical communication.
Meanwhile, the optical fiber collimator is used in the field of optical measurement. For example, in a fluorescence measurement device adapted to measure and evaluate fluorescence, which is generated by irradiating excitation light onto a substance, an optical filter is used to measure the intensity of fluorescence having a specific wavelength. Thus, a collimating function is needed (see, for instance, JP-A-11-326210). In such a fluorescence measurement device, excitation light is transmitted through an optical fiber close to a sample. Excitation light outputted from the optical fiber is condensed by a coupling lens. Then, the condensed light is irradiated onto the sample. Fluorescence generated therefrom by the irradiation of the excitation light is coupled by the coupling lens to a light receiving optical fiber. A wavelength component necessary for the measurement is extracted by the optical filter from the fluorescence transmitted from the optical fiber. Light outputted from the light receiving optical fiber is converted by a first collimator lens to parallel light beams. Then, the parallel light beams are incident upon the optical filter. The component transmitted by the optical filter is condensed by a second collimator lens. Subsequently, the condensed component is coupled to the optical fiber again and is led to a photodetector, such as a photomultiplier or an avalanche photodiode.
Recently, in the field of fluorescence measurement devices, systems using a bundle of a large number of multimode optical fibers, which is called “an optical fiber bundle”, as one optical transmission path have been increased to ensure a large diameter of an opening in the optical fiber and to assure flexibility required to treat an excess length part of each of optical fibers. However, the transmission system using such an optical fiber bundle has a problem that in the case of using the related optical fiber collimator, well-collimated light beams cannot be obtained.
The related optical fiber collimator is enabled to dispose the central axis of the collimator lens and that of the optical fiber on a same line. Fundamentally, a light beam outputted from the optical fiber can travel on the optical axis of the collimator lens. It has been sufficient to dispose the optical fiber and the collimator lens by taking only the conversion of the outputted light beam to parallel light beams into account. However, in a case where the central axis of an optical fiber 10a placed along the central axis of an optical fiber bundle 14 is set to coincide with that of a collimator lens 12 in the transmission system using an optical fiber bundle, as shown in FIG. 6B, the central axis of an optical fiber 10b placed at the periphery of the optical fiber bundle 14 cannot be set to coincide with that of the collimator lens 12. Thus, divergent light beams outputted from a large number of optical fibers 10b, which have optical axes shifted from the optical axis of the collimator lens 12 and are other than the central optical axis 10a, are converted by the collimator lens 12 to parallel light beams. Accordingly, even in a case where the divergent light beams outputted from each of the optical fibers 10b are converted to parallel light beams, a plurality of parallel light beams, the optical axis of each of which is shifted from the optical axis of the collimator lens 12, are generated. Consequently, well-collimated light cannot be obtained. Therefore, in the case of constituting a system in which an optical filter is inserted between a pair of collimator lenses, coupling loss is caused by light beams traveling obliquely with respect to the optical axis of each of the collimator lenses. Consequently, the optical coupling loss caused between a pair of collimator lenses is very large. Especially, in a case where the optical filter is a bandpass filter, the system has a poor cutoff characteristic, because the designed characteristic of a dielectric multilayer filter is deviated from a necessary cutoff characteristic for the light beams traveling obliquely with respect to the optical axis of the collimator lens.